Make possible insert html into strings

// Excuse my beginner's English

Hmm, what about making able put into `td` html?

Simple use case:
```puthon
  from flask_table import Table, Col
  class Item(object):
      def __init__(self, id, name):
          self.name = name
          self.id = id
          self.count = 1
          self.mark = '<input type="checkbox">' #< HTML HERE =====================
  class ItemTable(Table):
      id = Col('Код')
      name =  Col('Наименование')
      count = Col('Количество')
      mark  = Col('Отметка добавл.')

  items = [Item(1, 'aaa'),
           Item(2, 'bbb')]
  tab = ItemTable(items)
```